The present invention relates generally to electrodes for detecting electromyographic (EMG) signals of the laryngeal muscles, and more particularly to electrodes which are mounted on an endotracheal tube.
The recurrent laryngeal nerves, hereinafter referred to as the laryngeal nerves, course through the neck to the intrinsic laryngeal muscles, hereinafter referred to as the laryngeal muscles. There are two laryngeal nerves, one on the left side and one on the right of the neck. Each nerve controls a set of laryngeal muscles, including a vocal cord.
Damage to laryngeal nerves is a common complication of neck surgery. When the anatomy is relatively normal, the course of a laryngeal nerve along the neck is usually discernible. However, the presence of abnormal tissue, such as tumor, inflammation or trauma may make anatomic dissection of the nerve without damage nearly impossible.
If a laryngeal nerve is damaged during surgery, paralysis of the related laryngeal muscles can occur. Paralysis of the laryngeal muscles results in loss of speech and may also disrupt breathing by preventing air passage through the trachea. There is, therefore, a need for an apparatus which will aid in the location of the laryngeal nerves. It is also desirable to have an apparatus which warns a surgeon when contact is made with a laryngeal nerve.
During surgery a tube is usually placed through the patient's nose or mouth and into the trachea, passing between the sets of laryngeal muscles. This endotracheal tube is used to ventilate the lungs and may also be used to provide anesthesia. Most endotracheal tubes include an inflatable cuff surrounding the tube. Once the tube is inserted into the trachea, the cuff is inflated to prevent air from escaping by passing between the tube and the trachea wall.
One prior art method of locating a laryngeal nerve uses an endotracheal tube having an additional cuff located to be adjacent both sets of laryngeal muscles when the tube is placed in the trachea. See, e.g. Engel P.M., et al. "A Device For The Location and Protection of the Recurrent Laryngeal Nerve During Operations Upon the Neck." Surgery, Gynecology, and Obstetrics, 152:824-826, 1981. The additional cuff is inflated and connected to a device for detecting pressure changes. A probe delivering an electric charge is used to stimulate the laryngeal nerve. When the probe contacts the nerve, the related set of laryngeal muscles contract causing a pressure change within the cuff, which can be detected by the pressure sensing device. Thus a surgeon can locate a laryngeal nerve by using the probe to stimulate various portions of the neck until a response is noted on the pressure sensing device.
The pressure sensitive device, however, does not operate satisfactorily. It can only detect relatively large movements of the laryngeal muscles. Thus it may not be sensitive enough to register all responses in the muscles when the nerve is electrically stimulated.
The pressure sensitive device also may be unable to detect the indigenous electric discharge which occurs when a laryngeal nerve is manipulated. Laryngeal nerves emit an electrical impulse which travels to the related set of laryngeal muscles when the nerve is manipulated or is contacted by a surgical instrument. The impulse from contact or manipulation is generally smaller than that which can occur by use of outside electrical stimulation. Thus the pressure sensitive device may not be able to warn a surgeon when damage to a laryngeal nerve is imminent due to contact with a surgical instrument.
Another method of locating a laryngeal nerve is the use of electrodes emplaced directly into a related laryngeal muscle. See, e.g., Lipton R.J. et al., "Intraoperative Electrophysiologic Monitoring of Laryngeal Muscle During Thyroid Surgery." Laryngoscope, 98:1292-1296, 1988. These electrodes are connected to an EMG machine which measures changes in voltage in the muscle. A number of different electrode types are known in the art, including needles, needle pairs and hooked wires. These devices are capable of detecting electrical changes in a laryngeal muscle caused by external electrical stimulation of the related laryngeal nerve. These devices are also sensitive enough to detect the electrical changes which occur in a muscle when the related laryngeal nerve is stimulated by manipulation, and, thus can be used to inform a surgeon when contact is made with a laryngeal nerve. These devices are undesirable, however, because they are difficult to accurately emplace in the muscle. A medical technique requiring a high degree of expertise must be mastered by a surgeon before these electrodes can be used.
Another device for measuring EMG activity in the laryngeal muscles comprises a tube substantially thinner than an endotracheal tube, and containing electrode wires which extend from the tube's interior through the tube wall and then circumferentially around the tube. The tube is placed into the esophagus, which is located just behind the trachea. This tube monitors the posterior laryngeal muscles, the muscles at the back of the trachea, through the front or anterior esophagus wall. See e.g., Fujita et al., "A New Surface Electrode for Recording from the Posterior Cricoarytenoid Muscle". Laryngoscope, 99:316-320, 1989. The circumferential electrode device is designed for use while the patient is awake, to measure EMG activity during normal breathing and speaking, although it could also be used during surgery.
The circumferential electrode device is inadequate for use in locating and protecting the laryngeal nerve for a number of reasons. First, because each set of laryngeal muscles is located adjacent a respective right or left side of the trachea, the circumferential electrode device allows measurement of EMG signals from only one of the two sets of laryngeal muscles. More particularly, the device is thin and must be placed against either the right or left side of the anterior esophagus wall. Since the device contacts only one side of the anterior wall, it can only monitor the set of laryngeal muscles in the trachea closer to that one side. Second, placement of the thin tube is difficult because it requires that the thin tube be positioned in the esophagus without any direct visualization. Additionally, a circumferential electrode configuration could not be used on an endotracheal tube employed for ventilation during surgery, because that kind of tube is relatively thick; and a given electrode surrounding the thick endotracheal tube would unavoidably be in contact with both sets of laryngeal muscles at the same time.